As an effective agent for a variety of diseases, various triazolopyridazine compounds have been studied. For example, U.S. Pat. No. 3,915,968 corresponding to Japanese Patent Publication No. 7439/1978 discloses a compound of the following formula: ##STR2## wherein R and R.sup.3 respectively represent a hydrogen atom or a lower alkyl group (at least one of R and R.sup.3 is a lower alkyl group), R.sup.1 and R.sup.2, taken together with the nitrogen atom, represent a heterocyclic ring selected from pyrrolidine, piperidine, piperazine and morpholine, or a salt thereof.
U.S. Pat. No. 4,136,182 discloses a compound shown by the following formula: ##STR3## wherein R represents a hydrogen atom, phenyl, a lower alkyl or a lower alkyl carbonylamino; R.sup.1 represents morpholino or piperidino; and R.sup.2 represents a hydrogen atom or a lower alkyl, with the proviso that at least one of R and R.sup.2 is a group other than hydrogen atom and with the further proviso that when R is phenyl, R.sup.1 is morpholino and R.sup.2 is a lower alkyl, or a salt thereof.
EP-A-248413 corresponding to Japanese Patent Application Laid-open No. 292784/1987 discloses a compound shown by the following formula: ##STR4## or a salt thereof.
These literatures disclose that the compounds mentioned above are useful as bronchodilators which alleviate bronchospasm.
Although a variety of antiasthmatics have been launched into markets, no antiasthmatic satisfactory in effects, continuity of action, safety or other factors has been developed. Therefore, the development of a novel compound having more effective antiallergic, antiinflammatory and anti-PAF actions, and having excellent properties for an antiasthmatic agent such as action sustainability, safety and the like has been desired.